The ongoing research into the chemistry of absorbent materials, particularly fibrous materials such as cellulose, has yielded significant gains in the affectivity of absorbent products. Improvements in the absorptivity, fluid retention and other characteristics which affect user comfort, such as decreased stiffness, have all been realized. Additionally, it has been found that certain absorptive materials may be treated on line to render them hydrophobic or fluid repellent to some extent. Most absorbent products have at least one major surface which is exposed to a fluid flow or a collection of fluid. Hydrophobic materials may be combined with absorbent materials to provide a structure which absorbs and retains fluids, while preventing exudation of the retained fluids from the lateral sides of the pad, or by providing a layer which prevents the fluid from completely traversing the pad from the absorbing major surface to another surface.
Along with advanced materials and improved absorbent article structures comes the concomitant need for advanced manufacturing techniques. This is particularly true regarding items such as sanitary napkins, disposable diapers, wound dressings and the like, which are produced in necessarily extremely high volumes. In many instances, the materials which comprise such absorbent articles are manufactured in sheet form. In other instances, the materials may be in pellets or randomly sized pieces, however, they are usually not initially comprised of individual fibers. Accordingly, apparatus and methods have been developed whereby the starting materials are ground or otherwise physically impacted to render the material into a fibrous state, this process is known as "defiberizing". For example, wood pulp may be defiberized in a conventional hammermill, disk mill or lickerin. The defiberized material is then readily formed into absorbent articles having a high level of consistency.
It has been found that an efficient way of handling defiberized material for mass producing absorbent articles is by creating an air-entrained stream of the defiberized material. Using this technique, the material may be efficiently transported and handled in high speed production machinery. Air-entrained defiberized material may be formed into absorbent pads or the like by passing the airstream through a foraminous surface; if the foramen are of the proper size, the defiberized material is deposited thereon, this process is know as "airlaying". Most typically, apparatus may be constructed which comprises an air entrainment system which delivers a stream of fibers to a rotating drum. Disposed on the periphery of the drum are a series of foraminous cavities or molds, usually substantially in the shape of the desired article. In certain cases, however, the periphery is a continuous foraminous surface, allowing the production of a continuous airlaid sheet. Fibers are then airlaid on the foraminous surface; in some instances, if the cavity is overfilled, the excess is removed by passing the cavity in close proximity of a rotating brush, a process known as "scarfing". By regulating the internal pressure of the drum, relative to the pressure of the air-entrained stream, the density of the deposited fibers can be controlled. Additionally, in certain apparatus a positive pressure is created in portions of the drum, allowing the airlaid articles to be ejected for further processing.
An air-drum type apparatus for forming fibrous pads from a sheet of fibrous material is disclosed in U.S. Pat. No. 4,005,957--Savich. The apparatus described therein consists of a means for defiberizing a sheet of fibrous material and providing a stream of air-entrained fibers to a rotating drum. A series of foraminous cavities are disposed on the periphery of the drum, into which the air-entrained fibers are introduced, producing an airlaid article in the shape of the recess. Savich teaches the forming of the article in a curved recess and ejecting the airlaid article onto a flat vacuum conveyer, thereby providing an article with one curved side and one flat side. The pressure differential between the vacuum conveyor and the rotating drum is sufficient to compress the airlaid article into an article having a relatively greater basis weight.
Another example of airlaying apparatus is presented in U.S. Pat. No. 4,598,441--Stemmler, which discloses apparatus for the manufacture of absorbent pads shaped to fit the body. As discussed above, it is sometimes desireable to produce absorbent articles comprised of two or more materials. Stemmler teaches the use of what is essentially a series of two separate, identical forming machines connected by a transfer wheel. An air-entrained fiber stream is created from a cellulose web using separate apparatus attached to each machine. The first machine has a hollow drum having recesses into which a stream of air-entrained fibers is carried. The air-entrained fibers are deposited to form a first component layer of the absorbent article. The first layer of the article is then transferred to the second machine where the second layer of the article has been formed using a second stream of fibers. The two layers are pressed together by a transfer drum which transports the resulting two-layer article to be further compressed and ejected. Thus, Stemmler teaches the formation of a two layer article, each layer of which may be comprised of a different fibrous material. The reference teaches the placing of a first airlaid layer and a second airlaid layer of substantially the same shape and surface area atop one another, and compressing the layers to form an absorbent article.
An apparatus which incorporates apparatus for defiberizing a web of compressed material and forming the resulting fluff into absorbent pads is disclosed in U.S. Pat. No. 4,674,966--Johnson, et al. (Johnson I). A hammermill creates fluff, which is drawn by a vacuum shroud through insert molds which form the fluff into a desired shape. The molds are arrayed on a circular drum; any overfill is removed and recycled as each mold exits the drum. The molds illustrated are straight-sided and have a foraminous bottom surface, beneath which is disposed structure for controlling the airstream carrying the fibers.
The use of multiple streams of air-entrained fibers is taught by British Patent Application No. 2,191,793. The disclosure is primarily directed toward splitting streams of air-entrained fibers in a clump free manner. Such split streams are used to form absorbent articles having a central core of super absorbent, overlaid by a larger layer of fibers. It is important to keep the materials used for deposition free of clumps or other irregularities in order to assure a consistent product and an uninterrupted, continuous manufacturing process. The British reference teaches the use of two separate drum-type airlaying apparatus. In one of the airlaying drums, the air-entrained fibers are mixed with a superabsorbent prior to deposition in order to form the superabsorbent core. Separate pad layers produced by each airlaying apparatus are joined by compressing one atop the other using conveyor belts and other transfer apparatus external to the airlaying machines.
U.S. Pat. No. 4,592,708--Feist discloses apparatus for making airlaid articles of non-uniform thickness. The device disclosed therein is a drum-type airlaying apparatus for making discrete absorbent fibrous articles of airlaid matter. Feist utilizes a rotating deposition drum having a plurality of article formation cavities disposed in circumferentially spaced relation about the periphery of the deposition drum. Each of the cavities has a foraminous bottom wall. By providing means for directing air-entrained fibers toward the periphery of the drum and means for vacuum drawing the entrainment air through the foraminous bottom walls and exhausting it from the apparatus, discrete absorbent articles are obtained. The improvement provided by Feist consists of directing the stream of fibers substantially radially toward the drum, thus assuring deposition on the bottom of the forming cavity. Feist also teaches exhausting the entrainment air through substantially empty additional cavities disposed upstream from the cavity into which the fibers are being deposited.
In addition to airlaying apparatus generally, the control of the impinging air-entrained stream of fibers has been usefully exploited. It has been found that it is easier to regulate the deposition of airlaid fibers by regulating open area of the foraminous deposition surface and, concurrently or separately, regulating the pressure and/or velocity at or near the point of deposition. In this regard, it has been found that a variety of screens or baffles may be used to achieve certain useful conditions when such apparatus are placed beneath the foraminous deposition surface, that is, when the deposition surface is disposed between the impinging airstream and the regulating apparatus.
For example, U.S. Pat. No. 4,388,056--Lee et al. discloses apparatus for continuously making an airlaid fibrous web having a patterned basis weight distribution. The contoured web produced has alternately spaced relatively high basis weight and relatively low basis weight regions. Lee et al. teaches the use of air flow modulating means which can be adjusted to produce a range of pressure differentials across the foraminous bottom surface of the recess into which air-entrained fibers are introduced.
Also, West German Patent DE 3 5 08 344 A1 discloses a web forming assembly for use in a drum-type airlaying apparatus. A series of apertures are disposed along the bottom surface of the assembly, which is contoured to produce a pad having a central portion of a somewhat greater thickness than the remainder of the article formed. The cavities in the deposition drum are subdivided into a number of zones, each zone having a separate, regulated vacuum line. The device disclosed permits the production of airlaid articles of uniform quality having a varying thickness.
An apparatus and process having airflow regulating means is disclosed in U.S. Pat. No. 4,666,647--Enloe et al. (Enloe I). The improvement of Enloe I lies providing apparatus for the formation of laid fibrous articles having a non-stepwise gradation in basis weight. Enloe I teaches the use of a concave forming surface bounded by angled walls, whereby the laid fibrous article is readily removable from a foraminous forming surface. As shown in FIGS. 2 and 3 of Enloe I, the foramen are located on both the bottom and the sides of the recess into which the air-entrained fibers are laid.
Similarly, U.S. Pat. No. 4,761,258--Enloe (Enloe II) discloses apparatus for forming of airlaid fibrous webs. The webs disclosed have tailored absorbency zones, wherein certain areas have a higher basis weight, and therefore a higher absorbency. Enloe II teaches depositing air-entrained fibers on a foraminous web forming structure. A gas flow regulating means using apertures provides a selected pattern of gas flow resulting in a web forming assembly which creates a desired distribution of fiber basis weight across the formed web.
Finally, European Patent Application No. 85300626.0 (publication no. 151033 ) discloses methods and improved apparatus for making discrete airlaid absorbent articles. A drum-type apparatus is disclosed, having a series of cavities which have foraminous bottom walls. As shown in FIG. 2, the foraminous bottom wall is preferably a contoured mesh structure, having a relatively uniform distribution of the foramen. The airlaid articles are then further processed after deposition. The articles are compacted a predetermined amount and then ejected, the compaction means is shown as comprising a lugged wheel in gear-like engagement with the cavities of the airlaying drum.
Also known in the art is the separation of fluff components. For example, U.S. Pat. No. 4,625,552--Johnson (Johnson II) discloses an instrument for separating out any one or more of the components of any given sample of fluff. As pointed out by the reference, "fluff" is a collection of wood pulp fibers which, if recycled, during a production process may also include bits of reclamation waste, such as plastic or other materials. Although Johnson II is primarily directed toward testing apparatus for determining the composition of a sample of fluff, one of ordinary skill is also taught how to construct apparatus which can separate out any one or more of the components of the fluff. To accomplish this result, fluff is agitated by compressed air and loose fibers are separated and drawn through a sieve by a vacuum, while the non-fiber portions of the fluff are left in an agitation chamber. The fibers are caught in a sieve, while the highly defiberized, broken fibers are caught later downstream in a filter.
Thus, although several variations of airlaying apparatus and deposition control are known, it would be desireable to provide apparatus capable of continuously depositing two or more materials in selected areas of a product. At present, such constructions are limited to placing a first layer atop a second layer and compacting the resulting assembly. Such a construction is not preferred, however. In the case of many absorbent articles, such as those combining absorbent and repellent fibers. It would be further desireable that apparatus be constructed which required only a reduced the cost and complexity of the airlaying equipment, while still allowing two or more materials to be continuously deposited. Finally, such apparatus would ideally be flexible to the extent that the characteristics of the product and the geometry of the areas in which materials are selectively deposited may be easily changed.